Usually, a wheel bearing apparatus is used to freely rotationally support a wheel hub to mount the wheel, via a rolling bearing, for a driving wheel and a driven wheel. Generally, for structural reasons, an inner ring rotation type bearing is adopted for a driving wheel and both inner ring rotation and outer ring rotation types for a driven wheel. Double row angular ball bearings are widely used in such a bearing apparatus. The double row angular ball bearings has a desirable bearing rigidity, high durability against misalignment and small rotation torque, superior for fuel consumption. Double row tapered roller bearings are used for heavy weight vehicles such as off-road cars or trucks.
The vehicle bearing apparatus is broadly classified into a first generation structure where a wheel bearing of a double row angular contact ball bearing is fit between a knuckle, forming part of a suspension, and a wheel hub. In a second generation structure, a body mounting flange or a wheel mounting flange is formed directly on the outer circumference of an outer member. In a third generation structure, one of the inner raceway surfaces is directly formed on the outer circumference of the wheel hub. In a fourth generation structure, the inner raceway surfaces are directly formed on the outer circumference of the wheel hub and the constant velocity universal joint.
In the prior art wheel bearing apparatus, since both bearing row arrangements in double row bearing are the same, although it has a sufficient rigidity during straight way running, optimum rigidity cannot always be obtained during the curved way running. The positional relationship between the wheels and the bearing apparatus is usually designed so that the weight of the vehicle acts on substantially the middle between the rows of bearing balls during straight way running. However, during curved way running, a larger radial load and a larger axial load are applied to the axles of the vehicle of the side opposite to the curving direction (i.e., axles of the left hand side of vehicle when right hand curving). Accordingly, it is effective to have a larger rigidity of the bearing row on the outer side than that of the bearing row of the inner side in order to improve the durability and strength of the bearing apparatus. Thus, a known vehicle wheel bearing apparatus shown in FIG. 11 can have a high rigidity without enlargement of the bearing apparatus.
The vehicle wheel bearing apparatus 50 is formed with a double row angular ball bearing comprising an outer member 51 integrally formed with a body mounting flange 51c on its outer circumference. The body mounting flange 51c is to be mounted on a knuckle (not shown) of a vehicle. Its inner circumference includes double row outer raceway surfaces 51a, 51b. An inner member 55 includes a wheel hub 52 with a wheel mounting flange 53 integrally formed at one end for mounting to a wheel (not shown). One inner raceway surface 52a is formed on the wheel hub outer circumference opposite to one 51a of the double row outer raceway surfaces 51a, 51b. A cylindrical portion 52b axially extends from the inner raceway surface 52a. An inner ring 54 is fit onto the cylindrical portion 52b. The inner ring outer circumference includes the other inner raceway surface 54a opposite to the other raceway surface 51b of the double row outer raceway surfaces 51a, 51b. Double row balls 56, 57 are freely rollably contained between the outer raceway surfaces 51a, 51b and inner raceway surfaces 52a, 54a of the inner member 55. Cages 58, 59 rollably hold the balls 56, 57.
The inner ring 54 is axially immovably secured on the cylindrical portion 52b by a caulked portion 52c. The caulked portion 52c is formed by plastically deforming, radially outward, the cylindrical portion 52b of the wheel hub 52. Seals 60, 61 are mounted in annular openings formed between the outer member 51 and the inner member 55. The seals prevent leakage of grease contained within the bearing apparatus and the entry of rain water or dusts into the bearing apparatus from the outside.
A pitch circle diameter D1 of the outer side ball group 56 is set larger than a pitch circle diameter D2 of the inner side ball group 57. Accordingly, the diameter of the inner raceway surface 52a of the wheel hub 52 is larger the diameter of the inner raceway surface 54a of the inner ring 54. The outer raceway surface 51a of the outer side of the outer member 51 is larger than that of the outer raceway surface 51b of the inner side of the outer member 51. Also, the number of outer side balls 56 is larger than the number of the inner side balls 57. By setting the pitch circle diameter D1 of the outer side is larger than the pitch circle diameter D2 of the inner side (D1>D2), it is possible to obtain a large rigidity of the bearing apparatus 50 and thus to extend its life (see Japanese Laid-open Patent Publication No. 108449/2004).